The acute shortage of human organs for transplantation has stimulated exploration into the possibility of using non-human donor organs for xenotransplantation. Pigs are now regarded as the most likely species to serve as donors for clinical xenotransplantation. The first major immunological barrier to overcome to allow successful xenotransplantation is rejection by natural antibodies (NAB) present in humans that are directed toward the carbohydrate epitope Gal(alpha1-3)Gal(beta1-4)GIcNAc-R (alphaGal) on porcine tissues. Like humans, knockout mice (GTo) carrying a null mutation in the gene encoding the glucosyltransferase UDP galactose:beta-D- galactosyl-1,4-N-acetyI-D-glucosaminide alpha(1-3)galactosyltransferase (E.C. 2.4.1.151, hereafter referred to as alphaGT) lack alphaGal epitopes on all tissues and develop in their serum NAB reactive against alphaGal. We have shown previously in GTo mice that genetic engineering of bone marrow (BM) using retroviral transduction to establish molecular chimerism can be used to induce long-term stable tolerance to alphaGal and prevent antibody mediated hyperacute rejection of cardiac transplants. These results demonstrate that B cells producing alphaGal reactive antibodies are functionally eliminated from the immunological repertoire of animals that express the retrovirally transduced alphaGT gene in BM derived cells. The goal of this proposal is to test the hypothesis that establishment of molecular chimerism can be similarly used to induce tolerance to the alphaGal epitope in non-human primates. The specific aims are to: 1) Determine the effect of molecular chimerism on production of alphaGal reactive NAB in primates; and 2) Determine whether the induction of molecular chimerism leads to tolerance to alphaGal in primates and determine the mechanism. These studies should provide practical mechanistic information important for the field of xenotransplantation and to the application of gene therapy to induce B cell tolerance, and may also advance our understanding of self-nonself discrimination in shaping the B cell repertoire. Furthermore, these studies may be generally applicable for re-establishing B cell tolerance in individuals with autoimmunity.